This invention relates to an electron gun structure for use in an in-line type color picture tube or cathode ray tube.
An electron gun structure of this type comprises three electron guns arranged in-line that is a center gun coaxial with the tube axis and two side guns disposed on the opposite sides of the center gun, the guns emitting electron beams impinging upon three color phosphors coated on the inner surface of the face plate of the tube.
Electrodes comprising these guns and having the same functions are constructed integrally as far as possible. Usually the control electrodes, accelerating electrodes, focusing electrodes and anode electrodes constituting respective guns are constructed integrally whereas the heaters and cathode electrodes are construced independently and electrically insulated from each other.
In the electron gun structure of this type it is usual to construct such that a heater, and a cathode electrode which are independently provided for each gun, and a common control electrode G.sub.1, an accelerating electrode G.sub.2, a focusing electrode, and an anode electrode which are commonly provided for each gun are disposed in a predetermined order and that these elements are integrally supported by such insulating members as beads. The positional relationship between the cathode electrode and the control electrode plays an important roll in determining various characteristics of a color picture tube. For example, the spacing between the cathode electrode and the control electrode has a direct influence upon the cut-off voltage characteristic of the color picture tube such that the rise in the heating temperature of the cathode electrode causes the cathode current characteristics becomes unbalance among respective guns.
A solution of this problem is disclosed in Japanese Patent Publication No. 6183 of 1969 corresponding to Dutch Patent Application No. 65 13,665 filed on Oct. 22, 1965 wherein the control electrode has a cup shape into which the heater and the cathode electrode are snugly inserted so as to support the control electrode by an insulating member such as a bead together with other electrodes thus maintaining the control electrode and the cathode electrode in a fixed positional relationship.
Although such supporting construction can solve the aforementioned problem to some extent, there are the following problems to be solved.
More particularly, when predetermined voltages are applied to respective electrodes to operate the electron guns, as the time elapse, the control electrode is gradually heated by the heat of the heater with the result that the control electrode undergoes thermal expansion to depart from the cathode electrode. However, in an electron gun structure utilizing a cup shaped control electrode, one end of the bead support is secured to the other wall near the top of the control electrode so that the top surface of the control electrode does not deform uniformly. In other words, the top surface of the control electrode undergoes a minimum deformation near the side surface to which one end of the bead support is secured whereas the central portion of the top surface remote from the side surface undergoes a maximum deformation. Consequently, the central portion of the top surface of the control electrode most remote from the cathode electrode deforms into a convex form. As the control electrodes of the side guns are disposed on the opposite sides of the center gun and spaced apart from each other, they deform substantially in the same extent as the side wall of the control electrode to which the one end of the bead support is secured.
As is well known in the art there is the following relation among the spacing l.sub.1 between the cathode electrode and the control electrode G.sub.1 of an electron gun, the thickness l.sub.2 of the top surface of the control electrode G.sub.1, the spacing l.sub.3 between the top surface of the control electrode G.sub.1 and an accelerating electrode G.sub.2 confronting thereto, the opening diameter D of the control electrode G.sub.1, and the cut-off voltage E.sub.KCO of an electron beam. EQU E.sub.KCO =K D/l.sub.1 .times.l.sub.2 .times.l.sub.3 .times.E.sub.c2 ( 1)
where K is a proportionally constant, and E.sub.C2 is a voltage impressed upon the accelerating electrode G.sub.2.
Under these conditions, assume now that only the spacing l.sub.1 increases, the cut-off voltages E.sub.KCO would decrease as equation (1) teaches. In the operation of a conventional color picture tube, if only the cut-off voltage E.sub.KCO decreases while the control electrode is supplied with zero volt and the cathode electrode is biased positively, also the beam current I.sub.K would decrease. Thus, the beam current I.sub.K decreases as the spacing l.sub.1 increases. Thus, difference in the spacings between the control electrodes and the cathode electrodes of the central and side guns causes difference in the beam current I.sub.K of respective guns thus degrating the white color balance.
Even when the beam current I.sub.K is compensated for suitably designing the circuit, the following problem still remains.
FIG. 1 of the accompanying drawing shows one example of the transient characteristics of the beam current that is the cathode current of a prior art electron gun of a color picture tube.
When the beam current I.sub.K is set to a constant value I.sub.KO after the electrode gun has reached a sufficiently stable operating state (at a time t.sub.2), then the operation of the electron gun is stopped, the gun is cooled completely and thereafter operated again the beam current I.sub.K manifests transient characteristics as shown in FIG. 1, in which the ordinate respects the beam current I.sub.K and the abscissa the time. Curve I shows the transient chracteristic of the center gun and curve II that of the side guns.
As above pointed out, when the spacing l.sub.1 gradually increases due to the thermal expansion of the control electrode, as curves I and II show, the beam current I.sub.K gradually decreases towards the stable value at time t.sub.2 after building up to a maximum value at time t.sub.1. Also in the prior art electron gun structure, as the spacing l.sub.1 of the side guns varies greatly than that of the center gun as above described, the decrease in the beam current I.sub.K of the center gun is larger as can be noted from curve I so that when the beam currents I.sub.K of the center and side guns are adjusted to be the same at time t.sub.2, at time t.sub.1 the beam current I.sub.K of the center gun would be larger than those of the side guns. When the electron gun structure of the color picture tube is constructed to have such I.sub.K transient characteristics, even when the beam currents I.sub.K of respective guns were adjusted such that the image on the fluorescent screen of the tube will be white, the white color balance of the tube would be impared at the time of starting the operation of the tube whereby unwanted colors appear.